Decouples rigid object and articulation asset classes (#644)

# Description

Since we override a lot of the functions from RigidObject inside the
Articulation class, we don't need to rely on inheritance anymore.
Duplicacy in the code makes it easier to understand the two classes'
functionalities without severely added overhead from the maintenance
side. Moreover, conceptually, it can be motivated that the two are
independent concepts.

This MR decouples the rigid object and articulation concepts in the
framework.

## Type of change

- Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- This change requires a documentation update

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

---------
Co-authored-by: David Hoeller <dhoeller@nvidia.com>

source/extensions/omni.isaac.lab/config/extension.toml

 [package]
 
 # Note: Semantic Versioning is used: https://semver.org/
-version = "0.20.5"
+version = "0.20.6"
 
 # Description
 title = "Isaac Lab framework for Robot Learning"

source/extensions/omni.isaac.lab/docs/CHANGELOG.rst

 Changelog
 ---------
 
+0.20.6 (2024-08-02)
+~~~~~~~~~~~~~~~~~~~
+
+Changed
+^^^^^^^
+
+* Removed the hierarchy from :class:`~omni.isaac.lab.assets.RigidObject` class to
+  :class:`~omni.isaac.lab.assets.Articulation` class. Previously, the articulation class overrode  almost
+  all the functions of the rigid object class making the hierarchy redundant. Now, the articulation class
+  is a standalone class that does not inherit from the rigid object class. This does add some code
+  duplication but the simplicity and clarity of the code is improved.
+
+
 0.20.5 (2024-08-02)
 ~~~~~~~~~~~~~~~~~~~
 

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/articulation/articulation.py

@@ -24,14 +24,14 @@ import omni.isaac.lab.utils.math as math_utils
 import omni.isaac.lab.utils.string as string_utils
 from omni.isaac.lab.actuators import ActuatorBase, ActuatorBaseCfg, ImplicitActuator
 
-from ..rigid_object import RigidObject
+from ..asset_base import AssetBase
 from .articulation_data import ArticulationData
 
 if TYPE_CHECKING:
     from .articulation_cfg import ArticulationCfg
 
 
-class Articulation(RigidObject):
+class Articulation(AssetBase):
     """An articulation asset class.
 
     An articulation is a collection of rigid bodies connected by joints. The joints can be either
@@ -48,11 +48,6 @@ class Articulation(RigidObject):
     articulation root prim and creates the corresponding articulation in the physics engine. The
     articulation root prim can be specified using the :attr:`AssetBaseCfg.prim_path` attribute.
 
-    The articulation class is a subclass of the :class:`RigidObject` class. Therefore, it inherits
-    all the functionality of the rigid object class. In case of an articulation, the :attr:`root_physx_view`
-    attribute corresponds to the articulation root view and can be used to access the articulation
-    related data.
-
     The articulation class also provides the functionality to augment the simulation of an articulated
     system with custom actuator models. These models can either be explicit or implicit, as detailed in
     the :mod:`omni.isaac.lab.actuators` module. The actuator models are specified using the
@@ -111,6 +106,10 @@ class Articulation(RigidObject):
     def data(self) -> ArticulationData:
         return self._data
 
+    @property
+    def num_instances(self) -> int:
+        return self.root_physx_view.count
+
     @property
     def is_fixed_base(self) -> bool:
         """Whether the articulation is a fixed-base or floating-base system."""
@@ -160,22 +159,35 @@ class Articulation(RigidObject):
     """
 
     def reset(self, env_ids: Sequence[int] | None = None):
-        super().reset(env_ids)
         # use ellipses object to skip initial indices.
         if env_ids is None:
             env_ids = slice(None)
         # reset actuators
         for actuator in self.actuators.values():
             actuator.reset(env_ids)
+        # reset external wrench
+        self._external_force_b[env_ids] = 0.0
+        self._external_torque_b[env_ids] = 0.0
 
     def write_data_to_sim(self):
         """Write external wrenches and joint commands to the simulation.
 
         If any explicit actuators are present, then the actuator models are used to compute the
         joint commands. Otherwise, the joint commands are directly set into the simulation.
+
+        Note:
+            We write external wrench to the simulation here since this function is called before the simulation step.
+            This ensures that the external wrench is applied at every simulation step.
         """
-        # apply external forces and torques
-        super().write_data_to_sim()
+        # write external wrench
+        if self.has_external_wrench:
+            self.root_physx_view.apply_forces_and_torques_at_position(
+                force_data=self._external_force_b.view(-1, 3),
+                torque_data=self._external_torque_b.view(-1, 3),
+                position_data=None,
+                indices=self._ALL_INDICES,
+                is_global=False,
+            )
 
         # apply actuator models
         self._apply_actuator_model()
@@ -186,6 +198,28 @@ class Articulation(RigidObject):
             self.root_physx_view.set_dof_position_targets(self._joint_pos_target_sim, self._ALL_INDICES)
             self.root_physx_view.set_dof_velocity_targets(self._joint_vel_target_sim, self._ALL_INDICES)
 
+    def update(self, dt: float):
+        self._data.update(dt)
+
+    """
+    Operations - Finders.
+    """
+
+    def find_bodies(self, name_keys: str | Sequence[str], preserve_order: bool = False) -> tuple[list[int], list[str]]:
+        """Find bodies in the articulation based on the name keys.
+
+        Please check the :meth:`omni.isaac.lab.utils.string_utils.resolve_matching_names` function for more
+        information on the name matching.
+
+        Args:
+            name_keys: A regular expression or a list of regular expressions to match the body names.
+            preserve_order: Whether to preserve the order of the name keys in the output. Defaults to False.
+
+        Returns:
+            A tuple of lists containing the body indices and names.
+        """
+        return string_utils.resolve_matching_names(name_keys, self.body_names, preserve_order)
+
     def find_joints(
         self, name_keys: str | Sequence[str], joint_subset: list[str] | None = None, preserve_order: bool = False
     ) -> tuple[list[int], list[str]]:
@@ -236,7 +270,29 @@ class Articulation(RigidObject):
     Operations - Writers.
     """
 
+    def write_root_state_to_sim(self, root_state: torch.Tensor, env_ids: Sequence[int] | None = None):
+        """Set the root state over selected environment indices into the simulation.
+
+        The root state comprises of the cartesian position, quaternion orientation in (w, x, y, z), and linear
+        and angular velocity. All the quantities are in the simulation frame.
+
+        Args:
+            root_state: Root state in simulation frame. Shape is (len(env_ids), 13).
+            env_ids: Environment indices. If None, then all indices are used.
+        """
+        # set into simulation
+        self.write_root_pose_to_sim(root_state[:, :7], env_ids=env_ids)
+        self.write_root_velocity_to_sim(root_state[:, 7:], env_ids=env_ids)
+
     def write_root_pose_to_sim(self, root_pose: torch.Tensor, env_ids: Sequence[int] | None = None):
+        """Set the root pose over selected environment indices into the simulation.
+
+        The root pose comprises of the cartesian position and quaternion orientation in (w, x, y, z).
+
+        Args:
+            root_pose: Root poses in simulation frame. Shape is (len(env_ids), 7).
+            env_ids: Environment indices. If None, then all indices are used.
+        """
         # resolve all indices
         physx_env_ids = env_ids
         if env_ids is None:
@@ -252,6 +308,12 @@ class Articulation(RigidObject):
         self.root_physx_view.set_root_transforms(root_poses_xyzw, indices=physx_env_ids)
 
     def write_root_velocity_to_sim(self, root_velocity: torch.Tensor, env_ids: Sequence[int] | None = None):
+        """Set the root velocity over selected environment indices into the simulation.
+
+        Args:
+            root_velocity: Root velocities in simulation frame. Shape is (len(env_ids), 6).
+            env_ids: Environment indices. If None, then all indices are used.
+        """
         # resolve all indices
         physx_env_ids = env_ids
         if env_ids is None:
@@ -477,8 +539,69 @@ class Articulation(RigidObject):
         self.root_physx_view.set_dof_limits(self._data.joint_limits.cpu(), indices=physx_env_ids.cpu())
 
     """
-    Operations - State.
+    Operations - Setters.
+    """
+
+    def set_external_force_and_torque(
+        self,
+        forces: torch.Tensor,
+        torques: torch.Tensor,
+        body_ids: Sequence[int] | slice | None = None,
+        env_ids: Sequence[int] | None = None,
+    ):
+        """Set external force and torque to apply on the asset's bodies in their local frame.
+
+        For many applications, we want to keep the applied external force on rigid bodies constant over a period of
+        time (for instance, during the policy control). This function allows us to store the external force and torque
+        into buffers which are then applied to the simulation at every step.
+
+        .. caution::
+            If the function is called with empty forces and torques, then this function disables the application
+            of external wrench to the simulation.
+
+            .. code-block:: python
+
+                # example of disabling external wrench
+                asset.set_external_force_and_torque(forces=torch.zeros(0, 3), torques=torch.zeros(0, 3))
+
+        .. note::
+            This function does not apply the external wrench to the simulation. It only fills the buffers with
+            the desired values. To apply the external wrench, call the :meth:`write_data_to_sim` function
+            right before the simulation step.
+
+        Args:
+            forces: External forces in bodies' local frame. Shape is (len(env_ids), len(body_ids), 3).
+            torques: External torques in bodies' local frame. Shape is (len(env_ids), len(body_ids), 3).
+            body_ids: Body indices to apply external wrench to. Defaults to None (all bodies).
+            env_ids: Environment indices to apply external wrench to. Defaults to None (all instances).
         """
+        if forces.any() or torques.any():
+            self.has_external_wrench = True
+            # resolve all indices
+            # -- env_ids
+            if env_ids is None:
+                env_ids = self._ALL_INDICES
+            elif not isinstance(env_ids, torch.Tensor):
+                env_ids = torch.tensor(env_ids, dtype=torch.long, device=self.device)
+            # -- body_ids
+            if body_ids is None:
+                body_ids = torch.arange(self.num_bodies, dtype=torch.long, device=self.device)
+            elif isinstance(body_ids, slice):
+                body_ids = torch.arange(self.num_bodies, dtype=torch.long, device=self.device)[body_ids]
+            elif not isinstance(body_ids, torch.Tensor):
+                body_ids = torch.tensor(body_ids, dtype=torch.long, device=self.device)
+
+            # note: we need to do this complicated indexing since torch doesn't support multi-indexing
+            # create global body indices from env_ids and env_body_ids
+            # (env_id * total_bodies_per_env) + body_id
+            indices = body_ids.repeat(len(env_ids), 1) + env_ids.unsqueeze(1) * self.num_bodies
+            indices = indices.view(-1)
+            # set into internal buffers
+            # note: these are applied in the write_to_sim function
+            self._external_force_b.flatten(0, 1)[indices] = forces.flatten(0, 1)
+            self._external_torque_b.flatten(0, 1)[indices] = torques.flatten(0, 1)
+        else:
+            self.has_external_wrench = False
 
     def set_joint_position_target(
         self, target: torch.Tensor, joint_ids: Sequence[int] | slice | None = None, env_ids: Sequence[int] | None = None
@@ -812,12 +935,21 @@ class Articulation(RigidObject):
         self._log_articulation_joint_info()
 
     def _create_buffers(self):
-        # allocate buffers
-        super()._create_buffers()
+        # constants
+        self._ALL_INDICES = torch.arange(self.num_instances, dtype=torch.long, device=self.device)
+
+        # external forces and torques
+        self.has_external_wrench = False
+        self._external_force_b = torch.zeros((self.num_instances, self.num_bodies, 3), device=self.device)
+        self._external_torque_b = torch.zeros_like(self._external_force_b)
 
         # asset data
         # -- properties
         self._data.joint_names = self.joint_names
+        self._data.body_names = self.body_names
+
+        # -- bodies
+        self._data.default_mass = self.root_physx_view.get_masses().clone()
 
         # -- default joint state
         self._data.default_joint_pos = torch.zeros(self.num_instances, self.num_joints, device=self.device)
@@ -906,7 +1038,16 @@ class Articulation(RigidObject):
     def _process_cfg(self):
         """Post processing of configuration parameters."""
         # default state
-        super()._process_cfg()
+        # -- root state
+        # note: we cast to tuple to avoid torch/numpy type mismatch.
+        default_root_state = (
+            tuple(self.cfg.init_state.pos)
+            + tuple(self.cfg.init_state.rot)
+            + tuple(self.cfg.init_state.lin_vel)
+            + tuple(self.cfg.init_state.ang_vel)
+        )
+        default_root_state = torch.tensor(default_root_state, dtype=torch.float, device=self.device)
+        self._data.default_root_state = default_root_state.repeat(self.num_instances, 1)
         # -- joint state
         # joint pos
         indices_list, _, values_list = string_utils.resolve_matching_names_values(
@@ -923,6 +1064,18 @@ class Articulation(RigidObject):
         self._data.default_joint_limits = self.root_physx_view.get_dof_limits().to(device=self.device).clone()
         self._data.joint_limits = self._data.default_joint_limits.clone()
 
+    """
+    Internal simulation callbacks.
+    """
+
+    def _invalidate_initialize_callback(self, event):
+        """Invalidates the scene elements."""
+        # call parent
+        super()._invalidate_initialize_callback(event)
+        # set all existing views to None to invalidate them
+        self._physics_sim_view = None
+        self._root_physx_view = None
+
     """
     Internal helpers -- Actuators.
     """

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/articulation/articulation_cfg.py

@@ -8,21 +8,25 @@ from dataclasses import MISSING
 from omni.isaac.lab.actuators import ActuatorBaseCfg
 from omni.isaac.lab.utils import configclass
 
-from ..rigid_object import RigidObjectCfg
+from ..asset_base_cfg import AssetBaseCfg
 from .articulation import Articulation
 
 
 @configclass
-class ArticulationCfg(RigidObjectCfg):
+class ArticulationCfg(AssetBaseCfg):
     """Configuration parameters for an articulation."""
 
-    class_type: type = Articulation
-
     @configclass
-    class InitialStateCfg(RigidObjectCfg.InitialStateCfg):
+    class InitialStateCfg(AssetBaseCfg.InitialStateCfg):
         """Initial state of the articulation."""
 
-        # root position
+        # root velocity
+        lin_vel: tuple[float, float, float] = (0.0, 0.0, 0.0)
+        """Linear velocity of the root in simulation world frame. Defaults to (0.0, 0.0, 0.0)."""
+        ang_vel: tuple[float, float, float] = (0.0, 0.0, 0.0)
+        """Angular velocity of the root in simulation world frame. Defaults to (0.0, 0.0, 0.0)."""
+
+        # joint state
         joint_pos: dict[str, float] = {".*": 0.0}
         """Joint positions of the joints. Defaults to 0.0 for all joints."""
         joint_vel: dict[str, float] = {".*": 0.0}
@@ -32,10 +36,17 @@ class ArticulationCfg(RigidObjectCfg):
     # Initialize configurations.
     ##
 
+    class_type: type = Articulation
+
     init_state: InitialStateCfg = InitialStateCfg()
     """Initial state of the articulated object. Defaults to identity pose with zero velocity and zero joint state."""
+
     soft_joint_pos_limit_factor: float = 1.0
-    """Fraction specifying the range of DOF position limits (parsed from the asset) to use.
-    Defaults to 1.0."""
+    """Fraction specifying the range of DOF position limits (parsed from the asset) to use. Defaults to 1.0.
+
+    The joint position limits are scaled by this factor to allow for a limited range of motion.
+    This is accessible in the articulation data through :attr:`ArticulationData.soft_joint_pos_limits` attribute.
+    """
+
     actuators: dict[str, ActuatorBaseCfg] = MISSING
     """Actuators for the robot with corresponding joint names."""

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/articulation/articulation_data.py

@@ -4,20 +4,20 @@
 # SPDX-License-Identifier: BSD-3-Clause
 
 import torch
+import weakref
 
 import omni.physics.tensors.impl.api as physx
 
 import omni.isaac.lab.utils.math as math_utils
 from omni.isaac.lab.utils.buffers import TimestampedBuffer
 
-from ..rigid_object import RigidObjectData
 
-
-class ArticulationData(RigidObjectData):
+class ArticulationData:
     """Data container for an articulation.
 
-    This class extends the :class:`RigidObjectData` class to provide additional data for
-    an articulation mainly related to the joints and tendons.
+    This class contains the data for an articulation in the simulation. The data includes the state of
+    the root rigid body, the state of all the bodies in the articulation, and the joint state. The data is
+    stored in the simulation world frame unless otherwise specified.
 
     An articulation is comprised of multiple rigid bodies or links. For a rigid body, there are two frames
     of reference that are used:
@@ -30,28 +30,48 @@ class ArticulationData(RigidObjectData):
     can be interpreted as the link frame.
     """
 
-    _root_physx_view: physx.ArticulationView
-    """The root articulation view of the object.
+    def __init__(self, root_physx_view: physx.ArticulationView, device: str):
+        """Initializes the articulation data.
 
-    Note:
-        Internally, this is stored as a weak reference to avoid circular references between the asset class
-        and the data container. This is important to avoid memory leaks.
+        Args:
+            root_physx_view: The root articulation view.
+            device: The device used for processing.
         """
-
-    def __init__(self, root_physx_view: physx.ArticulationView, device: str):
-        # Initialize the parent class
-        super().__init__(root_physx_view, device)  # type: ignore
+        # Set the parameters
+        self.device = device
+        # Set the root articulation view
+        # note: this is stored as a weak reference to avoid circular references between the asset class
+        #  and the data container. This is important to avoid memory leaks.
+        self._root_physx_view: physx.ArticulationView = weakref.proxy(root_physx_view)
+
+        # Set initial time stamp
+        self._sim_timestamp = 0.0
+
+        # Obtain global physics sim view
+        physics_sim_view = physx.create_simulation_view("torch")
+        physics_sim_view.set_subspace_roots("/")
+        gravity = physics_sim_view.get_gravity()
+        # Convert to direction vector
+        gravity_dir = torch.tensor((gravity[0], gravity[1], gravity[2]), device=self.device)
+        gravity_dir = math_utils.normalize(gravity_dir.unsqueeze(0)).squeeze(0)
+
+        # Initialize constants
+        self.GRAVITY_VEC_W = gravity_dir.repeat(self._root_physx_view.count, 1)
+        self.FORWARD_VEC_B = torch.tensor((1.0, 0.0, 0.0), device=self.device).repeat(self._root_physx_view.count, 1)
 
         # Initialize history for finite differencing
         self._previous_joint_vel = self._root_physx_view.get_dof_velocities().clone()
 
         # Initialize the lazy buffers.
+        self._root_state_w = TimestampedBuffer()
         self._body_state_w = TimestampedBuffer()
+        self._body_acc_w = TimestampedBuffer()
         self._joint_pos = TimestampedBuffer()
         self._joint_acc = TimestampedBuffer()
         self._joint_vel = TimestampedBuffer()
 
     def update(self, dt: float):
+        # update the simulation timestamp
         self._sim_timestamp += dt
         # Trigger an update of the joint acceleration buffer at a higher frequency
         # since we do finite differencing.
@@ -61,6 +81,9 @@ class ArticulationData(RigidObjectData):
     # Names.
     ##
 
+    body_names: list[str] = None
+    """Body names in the order parsed by the simulation view."""
+
     joint_names: list[str] = None
     """Joint names in the order parsed by the simulation view."""
 
@@ -71,6 +94,16 @@ class ArticulationData(RigidObjectData):
     # Defaults.
     ##
 
+    default_root_state: torch.Tensor = None
+    """Default root state ``[pos, quat, lin_vel, ang_vel]`` in local environment frame. Shape is (num_instances, 13).
+
+    The position and quaternion are of the articulation root's actor frame. Meanwhile, the linear and angular
+    velocities are of its center of mass frame.
+    """
+
+    default_mass: torch.Tensor = None
+    """Default mass read from the simulation. Shape is (num_instances, num_bodies)."""
+
     default_joint_pos: torch.Tensor = None
     """Default joint positions of all joints. Shape is (num_instances, num_joints)."""
 
@@ -267,20 +300,20 @@ class ArticulationData(RigidObjectData):
         return self._body_acc_w.data
 
     @property
-    def body_lin_acc_w(self) -> torch.Tensor:
-        """Linear acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
-
-        This quantity is the linear acceleration of the articulation links' center of mass frame.
-        """
-        return self.body_acc_w[..., 0:3]
+    def projected_gravity_b(self):
+        """Projection of the gravity direction on base frame. Shape is (num_instances, 3)."""
+        return math_utils.quat_rotate_inverse(self.root_quat_w, self.GRAVITY_VEC_W)
 
     @property
-    def body_ang_acc_w(self) -> torch.Tensor:
-        """Angular acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+    def heading_w(self):
+        """Yaw heading of the base frame (in radians). Shape is (num_instances,).
 
-        This quantity is the angular acceleration of the articulation links' center of mass frame.
+        Note:
+            This quantity is computed by assuming that the forward-direction of the base
+            frame is along x-direction, i.e. :math:`(1, 0, 0)`.
         """
-        return self.body_acc_w[..., 3:6]
+        forward_w = math_utils.quat_apply(self.root_quat_w, self.FORWARD_VEC_B)
+        return torch.atan2(forward_w[:, 1], forward_w[:, 0])
 
     @property
     def joint_pos(self):
@@ -305,10 +338,128 @@ class ArticulationData(RigidObjectData):
         """Joint acceleration of all joints. Shape is (num_instances, num_joints)."""
         if self._joint_acc.timestamp < self._sim_timestamp:
             # note: we use finite differencing to compute acceleration
-            self._joint_acc.data = (self.joint_vel - self._previous_joint_vel) / (
-                self._sim_timestamp - self._joint_acc.timestamp
-            )
+            time_elapsed = self._sim_timestamp - self._joint_acc.timestamp
+            self._joint_acc.data = (self.joint_vel - self._previous_joint_vel) / time_elapsed
             self._joint_acc.timestamp = self._sim_timestamp
             # update the previous joint velocity
             self._previous_joint_vel[:] = self.joint_vel
         return self._joint_acc.data
+
+    ##
+    # Derived properties.
+    ##
+
+    @property
+    def root_pos_w(self) -> torch.Tensor:
+        """Root position in simulation world frame. Shape is (num_instances, 3).
+
+        This quantity is the position of the actor frame of the articulation root.
+        """
+        return self.root_state_w[:, :3]
+
+    @property
+    def root_quat_w(self) -> torch.Tensor:
+        """Root orientation (w, x, y, z) in simulation world frame. Shape is (num_instances, 4).
+
+        This quantity is the orientation of the actor frame of the articulation root.
+        """
+        return self.root_state_w[:, 3:7]
+
+    @property
+    def root_vel_w(self) -> torch.Tensor:
+        """Root velocity in simulation world frame. Shape is (num_instances, 6).
+
+        This quantity contains the linear and angular velocities of the articulation root's center of
+        mass frame.
+        """
+        return self.root_state_w[:, 7:13]
+
+    @property
+    def root_lin_vel_w(self) -> torch.Tensor:
+        """Root linear velocity in simulation world frame. Shape is (num_instances, 3).
+
+        This quantity is the linear velocity of the articulation root's center of mass frame.
+        """
+        return self.root_state_w[:, 7:10]
+
+    @property
+    def root_ang_vel_w(self) -> torch.Tensor:
+        """Root angular velocity in simulation world frame. Shape is (num_instances, 3).
+
+        This quantity is the angular velocity of the articulation root's center of mass frame.
+        """
+        return self.root_state_w[:, 10:13]
+
+    @property
+    def root_lin_vel_b(self) -> torch.Tensor:
+        """Root linear velocity in base frame. Shape is (num_instances, 3).
+
+        This quantity is the linear velocity of the articulation root's center of mass frame with
+        respect to the articulation root's actor frame.
+        """
+        return math_utils.quat_rotate_inverse(self.root_quat_w, self.root_lin_vel_w)
+
+    @property
+    def root_ang_vel_b(self) -> torch.Tensor:
+        """Root angular velocity in base world frame. Shape is (num_instances, 3).
+
+        This quantity is the angular velocity of the articulation root's center of mass frame with respect to the
+        articulation root's actor frame.
+        """
+        return math_utils.quat_rotate_inverse(self.root_quat_w, self.root_ang_vel_w)
+
+    @property
+    def body_pos_w(self) -> torch.Tensor:
+        """Positions of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the position of the rigid bodies' actor frame.
+        """
+        return self.body_state_w[..., :3]
+
+    @property
+    def body_quat_w(self) -> torch.Tensor:
+        """Orientation (w, x, y, z) of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 4).
+
+        This quantity is the orientation of the rigid bodies' actor frame.
+        """
+        return self.body_state_w[..., 3:7]
+
+    @property
+    def body_vel_w(self) -> torch.Tensor:
+        """Velocity of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 6).
+
+        This quantity contains the linear and angular velocities of the rigid bodies' center of mass frame.
+        """
+        return self.body_state_w[..., 7:13]
+
+    @property
+    def body_lin_vel_w(self) -> torch.Tensor:
+        """Linear velocity of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the linear velocity of the rigid bodies' center of mass frame.
+        """
+        return self.body_state_w[..., 7:10]
+
+    @property
+    def body_ang_vel_w(self) -> torch.Tensor:
+        """Angular velocity of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the angular velocity of the rigid bodies' center of mass frame.
+        """
+        return self.body_state_w[..., 10:13]
+
+    @property
+    def body_lin_acc_w(self) -> torch.Tensor:
+        """Linear acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the linear acceleration of the rigid bodies' center of mass frame.
+        """
+        return self.body_acc_w[..., 0:3]
+
+    @property
+    def body_ang_acc_w(self) -> torch.Tensor:
+        """Angular acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the angular acceleration of the rigid bodies' center of mass frame.
+        """
+        return self.body_acc_w[..., 3:6]

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/rigid_object/rigid_object.py

@@ -120,6 +120,10 @@ class RigidObject(AssetBase):
     def update(self, dt: float):
         self._data.update(dt)
 
+    """
+    Operations - Finders.
+    """
+
     def find_bodies(self, name_keys: str | Sequence[str], preserve_order: bool = False) -> tuple[list[int], list[str]]:
         """Find bodies in the articulation based on the name keys.
 

source/extensions/omni.isaac.lab/omni/isaac/lab/assets/rigid_object/rigid_object_data.py

@@ -33,24 +33,20 @@ class RigidObjectData:
     is older than the current simulation timestamp. The timestamp is updated whenever the data is updated.
     """
 
-    _root_physx_view: physx.RigidBodyView
-    """The root rigid body view of the object.
-
-    Note:
-        Internally, this is stored as a weak reference to avoid circular references between the asset class
-        and the data container. This is important to avoid memory leaks.
-    """
-
     def __init__(self, root_physx_view: physx.RigidBodyView, device: str):
         """Initializes the rigid object data.
 
         Args:
-            root_physx_view: The root rigid body view of the object.
+            root_physx_view: The root rigid body view.
             device: The device used for processing.
         """
         # Set the parameters
         self.device = device
-        self._root_physx_view = weakref.proxy(root_physx_view)  # weak reference to avoid circular references
+        # Set the root rigid body view
+        # note: this is stored as a weak reference to avoid circular references between the asset class
+        #  and the data container. This is important to avoid memory leaks.
+        self._root_physx_view: physx.RigidBodyView = weakref.proxy(root_physx_view)
+
         # Set initial time stamp
         self._sim_timestamp = 0.0
 
@@ -76,6 +72,7 @@ class RigidObjectData:
         Args:
             dt: The time step for the update. This must be a positive value.
         """
+        # update the simulation timestamp
         self._sim_timestamp += dt
 
     ##
@@ -97,7 +94,7 @@ class RigidObjectData:
     """
 
     default_mass: torch.Tensor = None
-    """ Default mass provided by simulation. Shape is (num_instances, num_bodies)."""
+    """Default mass read from the simulation. Shape is (num_instances, num_bodies)."""
 
     ##
     # Properties.
@@ -133,8 +130,7 @@ class RigidObjectData:
     def body_acc_w(self):
         """Acceleration of all bodies. Shape is (num_instances, 1, 6).
 
-        This quantity is the acceleration of the rigid bodies' center of mass frame. The acceleration
-        is computed using finite differencing of the linear and angular velocities of the bodies.
+        This quantity is the acceleration of the rigid bodies' center of mass frame.
         """
         if self._body_acc_w.timestamp < self._sim_timestamp:
             # note: we use finite differencing to compute acceleration
@@ -158,6 +154,10 @@ class RigidObjectData:
         forward_w = math_utils.quat_apply(self.root_quat_w, self.FORWARD_VEC_B)
         return torch.atan2(forward_w[:, 1], forward_w[:, 0])
 
+    ##
+    # Derived properties.
+    ##
+
     @property
     def root_pos_w(self) -> torch.Tensor:
         """Root position in simulation world frame. Shape is (num_instances, 3).
@@ -255,3 +255,19 @@ class RigidObjectData:
         This quantity is the angular velocity of the rigid bodies' center of mass frame.
         """
         return self.body_state_w[..., 10:13]
+
+    @property
+    def body_lin_acc_w(self) -> torch.Tensor:
+        """Linear acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the linear acceleration of the rigid bodies' center of mass frame.
+        """
+        return self.body_acc_w[..., 0:3]
+
+    @property
+    def body_ang_acc_w(self) -> torch.Tensor:
+        """Angular acceleration of all bodies in simulation world frame. Shape is (num_instances, num_bodies, 3).
+
+        This quantity is the angular acceleration of the rigid bodies' center of mass frame.
+        """
+        return self.body_acc_w[..., 3:6]